1. Technical Field
The present invention relates to damping force control apparatuses that control damping forces in vehicle suspension apparatuses.
2. Related Art
Vehicle suspension apparatuses include dampers and springs interposed between an sprung member (above-spring member) such as a vehicle body and unsprung members (below-spring members) such as lower arms linked to wheels. Suspension apparatuses configure a vibration system. Furthermore, suspension apparatuses function so as to dampen vibrations of the sprung member by damping forces generated by the dampers.
Damping force control apparatus, which controls the damping forces generated by the dampers to dampen various types of vibrations in the sprung member can be dampened, is known. The damping force control apparatus carries out damping control of vibrations in the sprung member by using a predetermined control theory such as Skyhook control, nonlinear H-infinity control, and so on.
Japanese Patent Application Publication No. 2001-1736 discloses a damping force control apparatus that controls a damping force generated by a damper by applying nonlinear H-infinity control to a control system which is designed based on the motion of a suspension apparatus expressed by a single-wheel model. This damping force control apparatus calculates a variable damping coefficient by solving a nonlinear H-infinity control problem. Notably, the variable damping coefficient represents a coefficient of a variable damping force relative to the vibration speed. The variable damping force is a variable component (nonlinear component) obtained by removing a linear component that changes linearly relative to the vibration speed (a linear damping force) from the total damping force generated by the damper. Furthermore, this damping force control apparatus calculates a requested damping force, which is a target damping force to be generated by the damper for control, based on a requested damping coefficient. The requested damping coefficient is obtained by adding the variable damping coefficient calculated as described above and a linear damping coefficient, which is a coefficient of the linear damping force relative to the vibration speed. The damping force characteristic of the damper is then controlled based on the calculated requested damping force.